Light emitting diode (LED) lighting systems are becoming more prevalent as replacements for existing lighting systems. LEDs are an example of solid state lighting (SSL) and have advantages over traditional lighting solutions such as incandescent and fluorescent lighting because they use less energy, are more durable, operate longer, can be combined in multi-color arrays that can be controlled to deliver virtually any color light, and contain no lead or mercury. In many applications, one or more LED chips (or dies) are mounted within an LED package or on an LED module, and such a device may make up part of a lighting unit, lamp, “light bulb” or more simply a “bulb,” which includes one or more power supplies to power the LEDs. An LED bulb may be made with a form factor that allows it to replace a standard threaded incandescent bulb, or any of various types of fluorescent lamps.
Care must be taken in the design of multi-chip LED devices. LEDs of different sizes have different current densities for the same drive current. As chips heat up, forward voltage drops, which, when chips are arranged in parallel will cause a chip to draw more current relative to its neighbors until current draw increases current density to a degree that forward voltage increases. The forward voltage distribution in some LED chips can hurt a multi-chip parallel arrangement as current draw will not be balanced, which can unbalance the optimal current/chip efficiency for a multi-chip device. It should be noted that large chips have a lower forward voltage for the same drive current than smaller chips due to current density.